The present invention relates to a technique for efficient starting and commutation of a multi-phase Brushless DC motor under all load conditions without rotor position sensors.
The Brushless DC (BLDC) motor is an inverted brush DC motor in construction. The armature coils in the brush dc motor are transferred to the stator in the BLDC motor and the fixed magnets in the brush DC motor are transferred to the rotor in the BLDC motor. This design eliminates the need for the mechanical commutators and brushes which function is executed by transistors which act as switches.
The motor is powered by turning on two transistors in the switching arrangement, which connect two of the three phase windings to the DC supply for ⅙ of the time in an electrical cycle. Rotation of the rotor magnets induces voltages in the three stator phase windings and efficient operation of this motor is ensured by energizing the two phase windings which are experiencing a constant BEMF. This ensures that most of the electrical input power is converted to mechanical power with the minimum copper loss in the process. The BEMF generated in the unenergized winding provides very useful information of rotor position, provided the energy which was stored in its magnetic field has been dissipated, thereby completing the commutation of the unenergized winding.
Since the BEMF of a winding is dependent on the rotor position, rotor position information must be known in order to energize a pair of stator windings. Rotor position information for the purpose of stator winding energization is obtained with the use of Hall Sensors in U.S. Pat. No. 3,783,359, Optical Encoders and Resolvers in sensored commutation techniques and with the use of the BEMF Zero Crossing in U.S. Pat. No. 4,027,215, BEMF Integration in U.S. Pat. No. 4,169,990 and BEMF Third Harmonic in U.S. Pat. No. 4,912,378 in sensorless commutation techniques. Many versions of the heretofore cited three sensorless commutation techniques have been developed using elaborate electronic circuitry and having some innovations over those said patents as aforementioned.
The Hall Sensor Commutation technique requires three Hall sensors and supporting components wired on a printed circuit board, a DC power supply, space in the motor body and a sensing magnet with similar polarity to that of the rotor magnet and properly aligned with the rotor magnet. Accurate positioning of the Hall sensors is required and five connection wires are required to supply rotor position information. This said commutation technique increases the cost, size and weight of the motor and decreases the reliability of the system.
Optical encoders and resolvers are connected to the motor shaft for providing rotor position information via electrical wires connected to them. Unlike the Hall sensors, these said encoders do not produce rotor position information at standstill. These two said rotor position information techniques add substantial cost to the motor and utilise one end of the motor shaft.
These said three sensorless commutation techniques, in addition to increasing the cost of the motor also reduce the reliability of motor operation either in failure of the sensing devices due to the harsh environment in which the sensors often operate or by damage to the electrical wires carrying the rotor position information.
The three sensorless commutation techniques as aforementioned have their individual drawbacks but they all suffer from one major drawback in that they are not self-starting and a starting technique must be employed to run the motor up to a speed where the BEMF is large enough for the sensorless technique to be implemented. In addition to these said sensorless techniques not being self-starting, they sometimes do not sense commutation points due to noise in the BEMF signal and are unable to operate properly at low speeds when the BEMF signal generated is small.
Starting methods have been presented in U.S. Pat. No. 4,694,210, U.S. Pat. No. 5,343,127 and U.S. Pat. No. 6,642,681 for sensorless commutation of Brushless DC motors. However, these said starting techniques are cumbersome and are different from the commutation technique used when the motor is running.